Touch panels for various applications have employed mechanical waves in a substrate as an alternative to other technologies used in touch position sensors such as infrared or light detection beams and resistive or capacitive arrays. Touch position panels based on the use of mechanical waves are primarily substrates in which an acoustic wave is propagated in the substrate and a touch at a position on the substrate results in absorption of at least a portion of the wave propagated in the substrate. By use of electronics, the position in an XY coordinate plane is determined and thus the position of the touch. The use of touch position panels has applications in the computer, pager, cellular phone, personal digital assistants and radio markets.
As stated, acoustic waves have been utilized in touch position panels in the past. To this end surface acoustic waves (SAW) have been utilized in the past and were seen to have certain drawbacks discussed below. Thereafter, bulk waves, preferably transverse acoustic waves were utilized in acoustic touch panels. The use of transverse or shear waves is well known in the art, and the advantages thereof are delineated in U.S. Pat. Nos. 5,243,148 and 5,329,070, the disclosures of which are specifically incorporated here in by reference. In the these referenced patents, the medium of propagation of the acoustic shear waves is preferably glass. Touch position sensors incorporating shear or transverse waves utilizing glass as the substrate has the attendant advantage of a substantially improved insensitivity to absorption by surface contaminants when compared to surface acoustic waves. The fact that surface acoustic waves are more readily absorbed than bulk waves, to include shear waves, has both advantages and disadvantages in applications to touch position panels. The fact that SAW waves are more readily absorbed has the clear disadvantage in that they are more susceptible to absorption by contaminant. However, the fact that SAW waves are more readily absorbed when compared to the absorption of shear waves results in an improved sensitivity, which is clearly desirable in touch panel applications. Fortunately, the relative insensitivity of bulk waves can be overcome electronically. As is delineated in the above incorporated patents to Knowles, shear waves can be generated and received with greater efficiencies than surface acoustic waves on glass substrates. The result is that the signal received and processed has a substantially greater signal-to-noise ratio when compared to surface acoustic waves. Accordingly, suitable electronics can be utilized to effect adequate signal processing of a shear wave in a touch position panel.
While it is true that shear waves propagating in a glass substrate provide dramatic improvements compared to surface acoustic waves, there are clear drawbacks to the use of glass as the substrate in various applications. To this end, the use of touch position sensors in portable devices to include readily transportable computers as well as radios, cellular phones, and pagers require a much more durable and less breakable touch position substrate for acoustic touch position substrate applications. In addition, one of the particular consideration that must be given in the design of an acoustic touch position sensor for the support of shear waves is the thickness of the substrate as clearly delineated in the '148 and '070 references recited above. To this end, the thickness of the touch position sensor is generally desired to be small enough so as to not support higher order modes and Lamb waves. However, it is often required to bond the substrate to another plate of material in order to provide the structural rigidity required. That is, a piece of glass used as a substrate that will support shear waves but will not readily support spurious modes may be too thin for almost any application and often it is required to bond the thin piece of glass to another plate so as to provide structural rigidity. In plastic, this consideration becomes less important as the intrinsic nature of plastics enable a greater degree of durability for a given thickness when compared to glass. Additionally, the weight of the substrate is to some extent a practical consideration in portable devices, and clearly the density of plastic is less than that of glass resulting in a lighter substrate, for a given size, which has attendant benefits in portable device as is readily obvious.
Unfortunately, the vast majority of plastic materials tested for use as a substrate in a three dimensional mechanical acoustic touch position sensor have losses that are not tolerable for commercial application. That is, while many plastic materials provide an attractive alternative to glass from the stand point of durability and weight in portable applications, the losses for materials such as acrylics and other plastics are unacceptably high at practical operating frequencies.
Accordingly, what is needed is a plastic substrate that will support shear waves in an acoustic touch position sensor having the durability of plastic and the attendant benefits thereof while having acceptable signal transmission levels at suitable frequencies.